Venetian type blinds are well-known window coverings. They typically consist of a headrail and a bottomrail and slats resting on ladders strung between the headrail and bottomrail. The bottomrail is raised and lowered by lift cords which run from the bottomrail through route holes in the slats and into the headrail. A cord lock is usually provided at one end of the headrail and the lift cords are routed through the cord lock. The blind is raised and lowered by pulling or releasing the lift cords. In another type of blind disclosed in my U.S. Pat. No. 5,573,051, the lift cords are routed through slots on the front edge or back edge of the slats. There are no route holes in the slats.
Venetian blinds are typically made from plastic or aluminum extrusions. In recent years wood blinds have become more popular. In wood blinds the slats are wood strips, wood composites or plastic formed to look like wood and the bottomrail and headrail may either be assembled from pieces of wood, wood laminated over a metal or plastic frame, or in the case of the bottomrail, a solid piece.
With the advent of double pane and other thermally insulated windows, builders are installing larger and larger windows in buildings. Most home windows do not exceed three feet in width. However, some homes have wider picture windows and multiple pane windows which together extend beyond three feet. Homeowners usually want to cover these windows with a single blind. There are also commercial buildings where the width of the window may be up to six feet or larger and the height equally as large. Some building owners desire to have a single venetian blind cover each of those large windows. Therefore, manufacturers now offer venetian blinds which are in excess of three feet in width. These large blinds can be quite heavy weighing over 20 pounds. About forty pounds of lift is required to raise a fully lowered 20 pound blind. The solution to lifting these blinds has been to provide additional lift cords, perhaps as many as one lift cord every eight to twelve inches along the width of the blind so that the weight is distributed over several cords. Because these blinds are so large, the lift cords are much longer than are found in blinds having a width of three feet or less. The lift cords are made from a braided material and will stretch. Consequently, I have noticed that in larger blinds the stretch will be unequal causing one end or the middle of the blind to sag or bow upward as the blinds are raised and lowered.
Another phenomenon which I have observed in very large blinds that I have recently made is that the descending cords move faster than the ascending cords. That is, if the cord is routed from an attachment point in the headrail to the bottomrail, though the bottomrail, back up into the headrail and out through the cord lock, the portion of the cord between the headrail and bottomrail and which goes to the cord lock would be descending and will move faster than that portion of the cord between the headrail and the bottomrail and which is attached to the headrail called the ascending portion. This occurs because the ascending portion tends to stretch more than the descending portion. Such stretching causes the bottom of the blind to bow as the blind is raised and lowered.
Consequently, there is a need for a lift system and particularly a cording arrangement which provides controlled lowering and raising of a heavy venetian blind without distortion.